In order to be adaptive to different filming positions, a cross arm 101 of a medical X-ray equipment is designed to rotate about a center shaft, i.e., to rotate on a plane that is vertical to the floor where the X-ray equipment is located. The most frequently used filming positions are chest position and decubitus position. When the chest position is to be taken, the cross arm 101 is at a horizontal position, and a tube 103 needs to be moved along a guide 102 on the cross arm to the farthest end, i.e., the distance from the tube 103 to a detector 104 (SID) is adjusted to maximum; when the decubitus position is to be taken, the cross arm is at a vertical position, the tube 103 needs to be moved along the guide 102 on the cross arm to the nearest end, i.e., the distance from the tube 103 to the detector 104 (SID) is adjusted to minimum, as shown in FIGS. 1A and 1B.
As for an existing manually-operated cross arm, a doctor needs to complete the above operation via the following two steps: first move a tube, adjust SID, and then rotate a cross arm. If the cross arm is rotated from a horizontal position to a vertical position first of all, SID cannot be adjusted for the reason that the position of the tube is too high.